This invention relates to overload protectors and more particularly to an overload protector which protects a tool activated by a programmable robot to prevent the tool from being overloaded or being forced into an abnormal position.
In Canadian patent no. 2,004,661 issued to Kaz M. Szmyr, one of the applicants herein, an overload protector is described in which the tool to be protected is attached to a flange which is located within the cavity of a housing. Fluid under pressure is contained within the cavity and the pressure of that fluid forces the flange into a wall of the cavity. There is an opening in that wall and when the flange is forced against the wall it seals the opening shut.
Movement of the tool caused by overloading or abnormal forces applied to it results in movement of the flange away from the wall. Such movement allows fluid within the cavity to escape through the opening and the pressure within the cavity drops. A pressure switch detects the drop in pressure and signals the robot to stop.
Should the tool be subject to torsion, the flange rotates relative to the wall of the housing. Ball bearings are provided in the inside wall of the flange and in the wall of the cavity that faces it. As the flange rotates, the ball bearings force the flange to move away from the hole and fluid within the cavity escapes.
The location of the ball bearings on the flange governs the amount of torsional force required to cause them to force the flange away from the cavity wall. The further those ball bearings are from the centre of the flange the more torsional force must be applied to the flange before the ball bearings force the flange away from the wall.
It has been found that cones can be substituted for ball bearings with improved results. The cones may be in the form of conical detents which are formed in the cavity wall and which are received in conical recesses at the periphery of the flange. Greater axial, angular and torsional movement or compliance can be achieved by this means than by ball bearings. The reason is that ball bearings will tend to roll out of their recesses should such movement be more than very limited and once out cannot be reset into their recesses without dismantling the protector. Conical detents and recesses, on the other hand, permit significantly more of such movement. Moreover should the detents separate from the recesses, the protector need not be taken apart to reset the detents into the recesses.
Conical detents and recesses have another advantage. Should unlimited rotational movement or torsional compliance be required, the detents can be shortened so that they will ride completely out of their recesses should a tool be subjected to a high speed collision. In such event the protector will have more time to signal the robot to stop and, after it has stopped, the detents can be easily reset into the recesses manually by an operator. Should the intensity of the collision be insufficient to cause the detents to ride out the recesses but sufficient to cause them to ride up in the recesses, a spring within the protector will reset the detents automatically into the proper position after the robot has stopped the tool and no manual resetting is necessary.
It has also been found that the sensitivity of the overload protector to axial forces applied to the tool can be enhanced by attaching to the flange an assembly which is slidable to a limited extent relative to the flange. The assembly is designed to allow fluid within the cavity to escape even though the axial force which overloads the tool is insufficient to overcome the pressure of the fluid on the flange.
According to one embodiment of the improvement, the flange has a circular peripheral wall having a plurality of semi-conical recesses formed therein. The cavity has semi-conical detents formed in its wall. The recesses and detents are arranged such that the detents are accommodated within the recesses when the flange assembly is in a sealing relationship with the wall of the cavity. Alternatively the cavity may have recesses in its wall and those recesses are received in detents in the periphery of the flange.
According to another embodiment of the improvement, a hub assembly is attached to the flange and is designed to slide a limited distance relative to the flange. The tool is operatively connected to the assembly such that when the tool is overloaded in such a way that it causes the assembly to advance toward the flange, fluid leaks from the cavity with resulting drop in the pressure of the fluid within the cavity.